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http://blog.koehntopp.de/uploads/chaum_fiat_naor_ecash.pdf
Untraceable Electronic Cash t
(Extended Abstract)
David Chum Amos Fiat * Moni Nmr3
Center for Mathematics and Computer Science
Kruislaan 413, 1098 SJ Amsterdam, The Netherlands
Tel-Aviv University
Tel-Aviv, Israel
IBM Almaden Research Center
650 Harry Road, San Jose, CA 95120
Introduction
The use of credit cards today is an act of faith on the pat of all concerned. Each party
is vulnerable to fraud by the others, and the cardholder in particular has no protection
against surveillance.
Paper cash is considered to have a significant advantage over credit cards with
respect to privacy, although the serial numbers on cash make it traceable in principle.
Chaum has introduced unconditionally untraceable electronic money( [C85] and [C88]).
But what is to prevent anyone from making several copies of an electronic coin and
using them at different shops? On-line clearing is one possible solution though a rather
expensive one. Paper banknotes don't present this problem, since making exact copies
of them is thought to be infeasible. Nor do credit cards, because their unique identity
lets the bank take legal action to regain overdrawn balances, and the bank can add
cards to a blacklist.
Generating an electronic cash should be difficult for anyone, unless it is done in
cooperation with the bank. The RSA digital signature scheme can be used to realize
untraceable electronic money as proposed in [C85 and C88]. This money might be
of the form (~,f(z>'/~ (mod n)) where n is some composite whose factorization is
known only to the bank and f is a suitable one-way function. The protocol for issuing
and spending such money can be summarized as follows:
1. Alice chooses arandom z and T, and supplies the bank with B = y3f(z) (mod n)).
t Work done while the second and third authors were at the University of California
at Berkeley. The work of the second author was supported by a Weizmann Postdoctoral
Fellowship and by NSF Grants DCR 84-11954 and DCR 85-13926. The work of the third
author was supported by NSF Grants DCR 85-13926 and CCR 88-13632.
S. Goldwasser (Ed.): Advances in Cryptology - CRYPT0 '88, LNCS 403, pp. 319-327, 1990.
0 Springer-Verlag Berlin Heidelberg 1990
320
2. The bank returns the third root of B modulo n: r . f(2)ll3 (mod n) and withdraws
one dollar from her account.
3. Alice extracts C = f(~)l/~ mod n from B.
4. To pay Bob one dollar, Alice gives him the pair (z,~(z)'/~
5. Bob immediately calls the bank, verifying that this electronic coin has not already
(mod n)).
been deposited.
Everyone can easily verify that the coin has the right structure and has been signed by
the bank, yet the bank cannot link this specific coin to Alice's account.
Among other advantages, the new approach presented here removes the requirement
that the shopkeeper must contact the bank during every transaction. If Alice
uses a coin only once, her privacy is protected unconditionally. But if Alice reuses a
coin, the bank can trace it to her account and can prove that she has used it twice.
Our work is motivated by that on minimum disclosure ([CSS], [BCSSa], [BC86b]
and [BCC]) and on zero-knowledge ([GMR], [GMWSSa] and [GMW86b]). Our scheme
protects Alice's privacy unconditionally as is possible with the former, rather than
computationally as in the latter. Using these very general results - which seem to be
infeasible in practice - the security of the protocols presented here could be reduced to,
say factoring (or any onw-way permutation if Alice's privacy is only computationally
secure). Instead, We use the cut-and-choose methodology (first introduced in [R77])
directly, yielding quite practical constructions.
The next section presents our basic scheme, which guarantees untraceability, yet
allows the bank to trace a "repeat spender". We then show how to modify the protocol
so that the bank can supply incontestable proof that Alice has reused her money.
Finally, we give a more efficient variant and briefly discuss further work.
1. Untraceable Coins
The bank initially publishes an RSA modulus n whose factorization is kept secret
and for which 9(n) has no small odd factors. The bank also sets some security
parameter k.
Let f and g be two-argument collision-free functions; that is, for any particular
such function, it is infeasible to find two inputs that map to the same point. We
require that f be "similar to a random oracle". For unconditional untraceability we
also require g to have the property that fixing the first argument gives a one-to-one (or
c to 1) map from the second argument onto the range.
Alice has a bank account numbered u and the bank keeps a counter v associated
with it. Let @ denote bitwise exclusive or and 11 denote concatenation.
To get an electronic coin, Alice conducts the following protocol with the bank:
1.
2.
3.
4.
5.
321
Alice chooses a;, c;, di and ri, 1 5 i 5 k, independently and uniformly at random
from the residues (mod n).
Alice forms and sends to the bank k blinded candidates (called B for mnemonic
purposes)
Bi=ri3.f(Zi,Yi)modn for I